In many applications, it is desirable, if not necessary, to monitor the operation of an alternating current dynamooelectric machine, such as a motor, to ensure that the motor is performing its functions properly and within normal operating ranges. For example, in modern high-density electronic circuitry for computers and the like, motor driven fans are employed to keep the termperature of the components within safe ranges. In some types of electronic equipment, the fans are positioned such that their operating condition is not readily observable, either visually or by sensing a flow of air; in others, where large racks of equipment are concentrated, such as in telephone switching centers, personnel are not available to make the requisite checks on the condition of the cooling equipment and failure of the latter will be evidenced only by breakdown of the switching apparatus.
A number of different expedients have been resorted to to provide an indication of whether or not a motor is operating. One such arrangement incorporates a magnet fixed to the rotor for rotation therewith and a magnetic sensing element, such as a Hall effect device, mounted on the stator such that each revolution of the rotor produces an output from the Hall device. The Hall device output is converted to a series of electrical pulses, the frequency of which is directly related to the speed of rotation of the motor. Pulse counting circuitry then converts the pulse frequency into a speed indication to inform any observer of the motor operation. This system has the drawback of requiring a separate magnet on the rotor which, especially in the case of small motors, introduces a balance problem and, further, requires relatively complex circuitry to perform the pulse counting and indicating functions.
Another type of sensing system, specifically adapted to fans, employs a vane or air movement responsive device in the path of the air flow. The vane is coupled to a switch which, in turn, is in circuit with a power supply and indicator. As long as air flow is above a minimum level, the vane is deflected to keep the switch open. Upon cessation of air flow, the switch closes and an alarm signal is given. This system, besides being applicable only to motor driven fans, is relatively cumbersome in that it requires external mechanical and electrical apparatus outside of the fan unit itself and, moreover, indicates only when the fan has essentially come to a complete stop, by which time it may be too late to avoid damage to the equipment being cooled.